UV Crosslinking Adhesive Technology and Its Advantages

UV Crosslinking Adhesives – Current Status and Applications

Although UV crosslinking adhesives have been around for 30 years, they are still considered a niche technology in the adhesive field. The goal of this technology is to achieve different adhesive properties using a single adhesive by adjusting the UV dose. This new technology aims to gradually replace solvent-based adhesives.

But what exactly is UV crosslinking? In chemistry, crosslinking refers to the process where macromolecular chains bond to form a three-dimensional network. This reaction changes the physical properties of the material, typically making it stiffer, harder, less soluble, and more thermally stable.

UV crosslinking adhesives cure when exposed to ultraviolet light. They are composed of polyacrylates containing benzophenone, which polymerizes under UV light. This photochemical process breaks double bonds in the photoinitiator, generating free radicals that immediately react with the acrylate molecules. The resulting molecules have longer chains and form the aforementioned three-dimensional network. This crosslinking occurs immediately after the adhesive is applied at temperatures of up to 140°C.

UVC

The process for UV acrylate hot melts uses UVC radiation in the 250-260 nm range. UVC radiation is distinguished from UVA and UVB by its shorter wavelength and the fact that it is not present in natural sunlight. This is why adhesives crosslinked using this radiation are resistant to aging and UV light.

UVC radiation is emitted by medium-pressure mercury lamps with typical power outputs of 120-240 W/cm, which are commonly used in the printing ink industry. The crosslinking process is very rapid due to the free radical reaction. The selected crosslinking dose controls the degree of crosslinking, which in turn determines the ratio of adhesion to cohesion, influencing the adhesive’s bonding performance.

However, short-wave UVC radiation can only penetrate the adhesive layer to a certain depth. The deeper the penetration, the more radiation is absorbed until none remains, resulting in no further crosslinking. Thus, the thickness of UV hot melt adhesive layers is limited.

Stable Doses

It is crucial to maintain a constant UV dose throughout the coating process, which requires regular monitoring. If the dose decreases (e.g., due to aging UV lamps or dust accumulation), the adhesive’s adhesion will increase, but its shear resistance will decrease. To maintain consistent adhesive performance in such cases, the lamp output power can be increased, or the conveyor belt speed can be slowed down.

Modern integrated lamp systems are equipped with measurement and control technology that can simultaneously measure UVC output and dose and automatically adjust based on preset values. Measuring instruments like the UVPowerMAP, Power Puck, or UVpad radiometers can be used for these checks.

Measuring UV Dose

The crosslinking reaction requires UVC light in the short wavelength range of 250-260 nm. The crosslinking dose controls the adhesion-to-cohesion ratio, which determines the adhesive’s bonding performance. The dose itself is determined by the lamp’s output power and the system speed.

UV dose is measured using a UV dosimeter capable of detecting UVC radiation, which is critical for crosslinking. Modern measurement instruments can detect UVA, UVB, UVC, and UVV radiation as well as their associated doses. Modern integrated lamp systems now come with measurement and control technology that continuously measures UVC output and dose and automatically corrects based on preset values.

Depending on the type of measurement instrument, measurements are taken at different bandwidths (spectral ranges). The wider the bandwidth, the higher the energy absorption. Some measurement instruments operate only within the 200-280 nm range, producing measurements up to seven times higher.

Diverse Applications

UV technology supports a wide range of applications, from ultra-removable to high-adhesion levels, which can be achieved by adjusting the base polymer formulation. UV crosslinked hot melts can also be used in moist environments.

They are the adhesive of choice for chemical companies in applications like labels and resealable packaging for wipes, where alcohol, oils, and emulsifiers may be present. These substances do not interfere with the resealing properties.

Other applications include washable bottle labels. For glass bottle recycling, labels must detach from bottles without leaving residue. Labels must also be easily removable from PET bottles during processes like shredding or submersion in solvent baths.

Typical tapes should not yellow and need to exhibit high adhesion to various surfaces, along with high levels of shear and temperature resistance. A specific application is high-adhesion labels containing important information, such as those used in chemical plants or on automotive batteries. Here, the labels are exposed to high temperatures and chemical exposure in engine compartments, including battery acid, engine oil, and gasoline.

Contact with Human Skin and Food

UV crosslinked hot melts can be formulated to receive approval for direct contact with skin. They typically do not cause skin irritation. Dressings containing this technology are more breathable, allowing them to be worn for long periods without wrinkling the skin.

Radiation-cured hot melts also meet strict standards for adhesives that come into direct or indirect contact with food. This is why labels printed in supermarkets can be applied directly to fruits and vegetables, for example. Additionally, this technology enables resealable packaging for biscuits, pasta, and cheese. Specifically, labels coated with UV hot melt adhesives can be safely applied to both wet and dry, non-greasy foods—a characteristic confirmed through migration studies by external laboratories.

100% Systems

Unlike dispersion and solvent-based adhesives, UV crosslinked adhesives are classified as hot melt products, often referred to as "100% systems." This means that only the adhesive is applied to the substrate; in other words, there are no substances that need to evaporate, dry, or be recovered.

The resulting reduction in material and weight translates into significant savings in transportation costs. Moreover, since no drying process is required, production speeds can be significantly increased, especially when applying thicker coatings.

Advantages at a Glance

UV crosslinking adhesives offer several advantages over traditional adhesives that cure via heat or humidity exposure. These include:

1. UV crosslinking adhesives cure within seconds, making them ideal for fast application.
2. Curing under UV light ensures high strength and strong adhesion.
3. No drying process is required, eliminating the need for drying equipment in factories.
4. UV crosslinked hot melts, composed of thermoplastic polymers, are resistant to various solvents and corrosive chemicals after radiation curing.
5. High temperature resistance: they can withstand up to 200°C for short exposures.
6. High UV resistance also makes them suitable for outdoor applications.
7. Certified for direct contact with food.
8. Suitable for applications involving contact with human skin.
9. No further crosslinking occurs after UVC exposure.

Should You Switch to UV Crosslinking Adhesives?

Switching from rubber-based hot melts to UV crosslinking alternatives is relatively easy, with only a few key points to consider. Anyone already processing rubber-based hot melts likely has most of the necessary equipment for handling radiation crosslinking products: a drum melter, buffer tank, and coater. In this case, the only upgrade required would be UV lamps capable of emitting in the UVC range.

Unlike application heads, which are relatively easy to clean, drum melters and hoses are very difficult to clean. Therefore, it's best to equip each technology with separate equipment; this also applies to buffer tanks.

On a more detailed level, a perfect UV investment ultimately depends on system breadth, operating speed, coating weight, space for installing one or more UV lamps, and the measurement and control technology used. Seeking good advice is definitely worthwhile!

Note: If you plan to use the same system for both technologies, be sure to account for the tedious cleaning process involved with each switch. Mixing radiation-cured hot melts with rubber-based products can lead to undesirable gelation, which may degrade quality in some cases.